DFB lasers have a diffraction grating etched along the length of the gain medium to form optical resonance cavities at desired wavelengths. Commonly available DFB lasers are made of layered indium phosphide or gallium arsenide compounds, and include a multi-quantum well active layer, or region, that is pumped to emit light at the wavelength determined by a diffraction grating in proximity to the active region. In high-speed optical communication systems, common DFB lasers typically operate in the 1310 nm or 1550 nm regions of the infrared spectrum.
In operation, DFB lasers are often combined with SOAs in order to amplify the optical output power. Monolithic integration, where the various components are grown on a single chip, is one way in which to combine them. It is desirable in a monolithically integrated DFB laser and SOA to have a high operational optical output power, in conjunction with a high side mode suppression ratio (SMSR) and narrow laser linewidth.
In a monolithically integrated DFB laser and SOA device, the optical amplifier section and the laser section may use the same active region design, except that the optical amplifier has no grating. By appropriate choice of the diffraction grating pitch, it is possible to adjust the laser linewidth by “detuning” the distributed feedback emission wavelength of the laser section from the material peak gain wavelength to an emission wavelength. Generally, the linewidth is decreased when selecting an emission wavelength situated on the short wavelength side of the gain peak wavelength. Because of this detuning, the laser now emits at a wavelength well removed from the gain peak (in the range of up to ˜30 nm from the gain peak wavelength). But, while it is possible to achieve a reduced linewidth by detuning the laser in this manner, because the laser and SOA share the same gain medium, the gain peak of the optical amplifier and emission wavelength of the laser no longer coincide. The latter effect compromises the peak optical output power and SMSR of the combined DFB laser and SOA.
It has also been found that minute back-reflections from the SOA output facet cause an unacceptable broadening of the laser linewidth. Techniques can be applied that reduce the optical reflections from the output of the SOA, thereby reducing the impact of facet reflections on the laser linewidth. These include: using an output waveguide that is angled relative to the plane of the output facet; using an anti-reflection coating at the output facet; using a flared waveguide; using a window region; and using a spot-size converter to increase the size of the optical mode at the output. More than one of these methods can be used in combination when attempting to minimize the reflectivity of the SOA output facet. However, it has been found that reflectivity reduction is not sufficient to maintain an adequately narrow linewidth, unless it is combined with substantial detuning, which, as stated above, is what limits the output power.
Attempts to attain greater output power by lengthening the SOA result in even greater degradation of the SMSR and the linewidth of the integrated device for two reasons. First, the SMSR of the combined laser and amplifier declines because there is a larger accumulation of spontaneous emission within the optical amplifier section, and second, the longer optical amplifier section allows for more amplification of the back-reflections, which then further degrade the linewidth of the laser. Even without amplification, the increased length of this second device results in a degradation of the laser linewidth because the larger volume stores more energy per round-trip, leading to increased laser instability.
It is, therefore, desirable to provide a monolithically integrated DFB laser and SOA optical device that has high power output, strong side mode suppression and adequately narrow linewidth, by using an efficient SOA that is as short as possible for a given output power performance target.